I know this is a really old thread but I had this problem with my 170 (which I still use today in 2017!) and for anyone searching now, they'll know what to do. The solution applies to many machines.
The solution is to install a relay that jumps the battery voltage at the starter directly to the solenoid, right at the starter. The solenoid pulls a lot of current and it doesn't take much series resistance to cause the solenoid not to fully engage and get the starter motor running. The keyswitch for instance is in series and over time develops a few ohms resistance. Battery connections too can cause the problem. Go to start, and just a click, click, click. Maybe if you're lucky, after several clicks, it works.
JD recognized the problem with the electrical design and came out with the AM107421 "Starting Improvement Relay Kit". If you look it up, you'll note they show several machines to use it with. In a nutshell, the wire providing power to the solenoid is used to just power the relay coil which does not require nearly as much current as the solenoid does. The N.O. (normally open) relay contacts connect the battery terminal of the starter directly to the solenoid. Works every time!!!
I did this fix on mine 3 years ago and I've never had a click since.